Semiconductor device

ABSTRACT

An object is to provide a semiconductor device with a novel structure in which stored data can be held even when power is not supplied and there is no limit on the number of write operations. The semiconductor device includes a first memory cell including a first transistor and a second transistor, a second memory cell including a third transistor and a fourth transistor, and a driver circuit. The first transistor and the second transistor overlap at least partly with each other. The third transistor and the fourth transistor overlap at least partly with each other. The second memory cell is provided over the first memory cell. The first transistor includes a first semiconductor material. The second transistor, the third transistor, and the fourth transistor include a second semiconductor material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/767,904, filed Feb. 15, 2013, now allowed, which is a is acontinuation of U.S. application Ser. No. 13/169,544, filed Jun. 27,2011, now U.S. Pat. No. 8,378,403, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2010-152021 on Jul. 2,2010, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device using asemiconductor element and a method for manufacturing the semiconductordevice.

BACKGROUND ART

Memory devices using semiconductor elements are broadly classified intotwo categories: a volatile device that loses stored data when powersupply stops, and a nonvolatile device that holds stored data even whenpower is not supplied.

A typical example of a volatile memory device is a dynamic random accessmemory (DRAM). A DRAM stores data in such a manner that a transistorincluded in a memory element is selected and charge is stored in acapacitor.

When data is read from a DRAM, charge in a capacitor is lost accordingto the above principle; thus, another write operation is necessary everytime data is read. Moreover, a transistor included in a memory elementhas leakage current (off-state current) between a source and a drain inan off state, or the like and charge flows into or out of a capacitoreven if the transistor is not selected, which makes a data holdingperiod short. For that reason, another write operation (refreshoperation) is necessary at predetermined intervals, and it is difficultto sufficiently reduce power consumption. Further, since stored data islost when power supply stops, another memory device using a magneticmaterial or an optical material is needed to hold stored data for a longtime.

Another example of a volatile memory device is a static random accessmemory (SRAM). An SRAM holds stored data by using a circuit such as aflip-flop and thus does not need refresh operation, which is anadvantage over a DRAM. However, cost per storage capacity is highbecause a circuit such as a flip-flop is used. Moreover, as in a DRAM,stored data in an SRAM is lost when power supply stops.

A typical example of a nonvolatile memory device is a flash memory. Aflash memory includes a floating gate between a gate electrode and achannel formation region of a transistor and stores data by holdingcharge in the floating gate. Therefore, a flash memory has advantages inthat the data holding period is extremely long (almost permanent) andrefresh operation which is necessary in a volatile memory device is notneeded (e.g., see Patent Document 1).

However, a gate insulating layer included in a memory elementdeteriorates owing to tunneling current generated in writing, so thatthe memory element stops its function after a certain number of writeoperations. In order to reduce adverse effects of this problem, a methodin which the number of write operations for memory elements is equalizedis employed, for example. However, a complicated peripheral circuit isneeded to realize this method. Moreover, even when such a method isemployed, the fundamental problem of lifetime is not solved. In otherwords, a flash memory is not suitable for applications in which data isfrequently rewritten.

In addition, a flash memory needs high voltage for holding charge in thefloating gate or removing charge from the floating gate, and also needsa circuit for generating high voltage. Further, it takes a relativelylong time to hold or remove charge, and it is not easy to performwriting and erasing at higher speed.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. S57-105889

DISCLOSURE OF INVENTION

In view of the above problems, an object of one embodiment of thepresent invention is to provide a semiconductor device with a novelstructure in which stored data can be held even when power is notsupplied and there is no limit on the number of write operations.Another object is to increase the integration degree of thesemiconductor device with a novel structure.

In one embodiment of the present invention, a semiconductor device ismanufactured using an oxide semiconductor. In particular, a highlypurified oxide semiconductor is used. A transistor including an oxidesemiconductor has extremely low leakage current; therefore, data can beheld for a long time. In the case of using a highly purified oxidesemiconductor, leakage current is much lower and thus data can be heldfor an extremely long time.

Specifically, the following structure can be employed, for example.

One embodiment of the present invention is a semiconductor deviceincluding a memory cell array including a first memory cell and a secondmemory cell, and a driver circuit driving the first memory cell and thesecond memory cell. The first memory cell includes a first transistorone of whose source and drain is connected to a first bit line and theother of whose source and drain is connected to a first source line, anda second transistor connected to a gate of the first transistor. Thesecond memory cell includes a third transistor one of whose source anddrain is connected to a second bit line and the other of whose sourceand drain is connected to a second source line, and a fourth transistorconnected to a gate of the third transistor. A semiconductor materialincluded in a channel formation region of the first transistor isdifferent from a semiconductor material included in channel formationregions of the second to fourth transistors. The first memory cell andthe second memory cell are stacked so as to overlap at least partly witheach other.

In the above structure, the semiconductor material included in thechannel formation region of the first transistor preferably includes asemiconductor material other than an oxide semiconductor. In the abovestructure, the semiconductor material included in the channel formationregions of the second to fourth transistors preferably includes an oxidesemiconductor material.

In the above structure, a part of the driver circuit preferably includesthe semiconductor material included in the channel formation region ofthe first transistor. In the above structure, a part of the drivercircuit preferably includes the semiconductor material included in thechannel formation regions of the second to fourth transistors.

In the above structure, it is preferable that a part of the drivercircuit include the semiconductor material included in the channelformation region of the first transistor, and that another part of thedriver circuit include the semiconductor material included in thechannel formation regions of the second to fourth transistors.

In the above structure, it is preferable that the first source line beelectrically connected to the second source line. In the abovestructure, it is preferable that the driver circuit include a selectorcircuit selecting the first memory cell or the second memory cell. Inthe above structure, it is preferable that the first bit line and thesecond bit line be electrically connected to the selector circuit.

Note that although the transistor is formed using an oxide semiconductormaterial in the above structure, one embodiment of the present inventionis not limited to this. A material which can realize off-state currentcharacteristics comparable to those of the oxide semiconductor material,such as a wide-gap material like silicon carbide (specifically, asemiconductor material with an energy gap Eg of greater than 3 eV, forexample), or the like may be used.

Note that in this specification and the like, the term such as “over” or“below” does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode over a gate insulating layer” can mean the case wherethere is an additional component between the gate insulating layer andthe gate electrode. Moreover, the terms such as “over” and “below” aresimply used for convenience of explanation.

In addition, in this specification and the like, the term such as“electrode” or “wiring” does not limit a function of a component. Forexample, an “electrode” is sometimes used as part of a “wiring”, andvice versa. Furthermore, the term such as “electrode” or “wiring” caninclude the case where a plurality of “electrodes” or “wirings” isformed in an integrated manner.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flow is changed in circuit operation, for example.Therefore, the terms “source” and “drain” can be used to denote thedrain and the source, respectively, in this specification and the like.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through an“object having any electric function”. There is no particular limitationon an “object having any electric function” as long as electric signalscan be transmitted and received between components that are connectedthrough the object.

Examples of the “object having any electric function” are a switchingelement such as a transistor, a resistor, an inductor, a capacitor, andan element having a variety of functions as well as an electrode and awiring.

Since the off-state current of a transistor including an oxidesemiconductor is extremely low, stored data can be held for a long timeby using the transistor. In other words, power consumption can besufficiently reduced because refresh operation becomes unnecessary orthe frequency of refresh operation can be extremely low. Moreover,stored data can be held for a long time even when power is not supplied.

Further, a semiconductor device according to one embodiment of thepresent invention does not need high voltage for data writing, and thereis no problem of deterioration of an element. For example, since thereis no need to perform injection of electrons into a floating gate andextraction of electrons from the floating gate which are needed in aconventional nonvolatile memory, deterioration of a gate insulatinglayer does not occur. That is, the semiconductor device according to oneembodiment of the present invention does not have a limit on the numberof times of rewriting, which has been a problem of a conventionalnonvolatile memory, and thus has significantly improved reliability.Furthermore, data is written by turning on or off the transistor,whereby high-speed operation can be easily realized. In addition, thereis an advantage in that operation for erasing data is not needed.

Since a transistor including a material other than an oxidesemiconductor can operate at sufficiently high speed, a semiconductordevice in which the transistor is used in combination with a transistorincluding an oxide semiconductor can perform operation (e.g., datareading) at sufficiently high speed. Further, with the transistorincluding a material other than an oxide semiconductor, a variety ofcircuits (such as a logic circuit or a driver circuit) which is requiredto operate at high speed can be favorably realized.

Thus, a semiconductor device having a novel feature can be realized bybeing provided with both the transistor including a material other thanan oxide semiconductor (a transistor capable of operation atsufficiently high speed, in general) and the transistor including anoxide semiconductor (a transistor whose off-state current issufficiently low, in general).

Moreover, in one embodiment of the present invention, a memory cell or adriver circuit partly has a stacked structure, whereby a semiconductordevice whose integration degree is improved can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view of a semiconductor device;

FIGS. 2A-1, 2A-2, 2B, and 2C are circuit diagrams of a semiconductordevice;

FIG. 3 is a block diagram of a semiconductor device;

FIG. 4 is a block diagram of a semiconductor device;

FIG. 5 is a circuit diagram of a semiconductor device;

FIG. 6 is a circuit diagram of a semiconductor device;

FIG. 7 is a circuit diagram of a semiconductor device;

FIG. 8 is a circuit diagram of a semiconductor device;

FIG. 9 is a circuit diagram of a semiconductor device;

FIG. 10 is a block diagram of a semiconductor device;

FIG. 11 is a circuit diagram of a semiconductor device;

FIG. 12 is a circuit diagram of a semiconductor device;

FIG. 13 is a block diagram of a semiconductor device;

FIG. 14 is a circuit diagram of a semiconductor device;

FIG. 15 is a circuit diagram of a semiconductor device;

FIGS. 16A to 16D are cross-sectional views illustrating a manufacturingprocess of a semiconductor device;

FIGS. 17A to 17D are cross-sectional views illustrating a manufacturingprocess of a semiconductor device;

FIGS. 18A to 18D are cross-sectional views illustrating a manufacturingprocess of a semiconductor device;

FIGS. 19A and 19B are cross-sectional views illustrating a manufacturingprocess of a semiconductor device; and

FIGS. 20A to 20F are views each illustrating an electronic deviceincluding a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the following description and it is easily understood bythose skilled in the art that modes and details can be modified invarious ways without departing from the spirit and scope of the presentinvention. Therefore, the present invention is not construed as beinglimited to the description of the following embodiments.

Note that the position, size, range, or the like of each structureillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, one embodiment of thepresent invention is not necessarily limited to the position, size,range, or the like as disclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Embodiment 1

In this embodiment, a structure and a manufacturing method of asemiconductor device according to one embodiment of the presentinvention will be described with reference to FIG. 1, FIGS. 2A-1, 2A-2,2B, and 2C, FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG.10, FIG. 11, FIG. 12, FIG. 13, FIG. 14, FIG. 15, FIGS. 16A to 16D, FIGS.17A to 17D, FIGS. 18A to 18D, and FIGS. 19A and 19B.

<Cross-Sectional Structure of Semiconductor Device>

FIG. 1 illustrates a cross section of a semiconductor device. In FIG. 1,A1-A2 is a cross section perpendicular to a channel length direction ofa transistor, and B1-B2 is a cross section parallel to the channellength direction of the transistor. The semiconductor device illustratedin FIG. 1 includes a first stack 210 a and a second stack 210 b. In thefirst stack 210 a, a transistor 160 including a first semiconductormaterial is provided in a lower portion, and a transistor 162 includinga second semiconductor material is provided in an upper portion. In thesecond stack 210 b, a transistor 170 including the second semiconductormaterial is provided in a lower portion, and a transistor 172 includingthe second semiconductor material is provided in an upper portion. Inthe first stack 210 a, a stack 213 a corresponds to a layer whichincludes the transistor 160 including the first semiconductor material,and a stack 213 b corresponds to a layer which includes the transistor162 including the second semiconductor material. In the second stack 210b, a stack 213 c corresponds to a layer which includes the transistor170 including the second semiconductor material, and a stack 213 dcorresponds to a layer which includes the transistor 172 including thesecond semiconductor material.

FIG. 1 illustrates a structure in which the first stack 210 a includesone transistor 160, one transistor 162, and one capacitor 164; however,the first stack 210 a may include a plurality of transistors 160, aplurality of transistors 162, and a plurality of capacitors 164.Similarly, the second stack 210 b includes one transistor 170, onetransistor 172, and one capacitor 174; however, the second stack 210 bmay include a plurality of transistors 170, a plurality of transistors172, and a plurality of capacitors 174.

Here, the first semiconductor material is preferably different from thesecond semiconductor material. For example, a semiconductor materialother than an oxide semiconductor can be used as the first semiconductormaterial, and an oxide semiconductor can be used as the secondsemiconductor material. The semiconductor material other than an oxidesemiconductor can be, for example, silicon, germanium, silicongermanium, silicon carbide, or gallium arsenide, and a single crystalsemiconductor is preferably used. A transistor including such asemiconductor material can operate at high speed easily. Alternatively,an organic semiconductor material or the like may be used. On the otherhand, a transistor including an oxide semiconductor can hold charge fora long time owing to its characteristics.

Either an n-channel transistor or a p-channel transistor can be employedas the transistor 160, the transistor 162, the transistor 170, and thetransistor 172. Here, the case where the transistor 160, the transistor162, the transistor 170, and the transistor 172 are all re-channeltransistors is described. The technical feature of one embodiment of thepresent invention is to use a semiconductor material with whichoff-state current can be sufficiently reduced, such as an oxidesemiconductor, in the transistor 162 and the transistor 172 in order tohold data. Therefore, it is not necessary to limit specific conditionssuch as a material, a structure, and the like of the semiconductordevice to those given here.

The transistor 160 includes a channel formation region 116 provided in asubstrate 100 including a semiconductor material (e.g., silicon),impurity regions 120 provided such that the channel formation region 116is sandwiched therebetween, metal compound regions 124 in contact withthe impurity regions 120, a gate insulating layer 108 provided over thechannel formation region 116, and a gate electrode 110 provided over thegate insulating layer 108. Note that a transistor whose source electrodeand drain electrode are not illustrated in a drawing may be referred toas a transistor for the sake of convenience. Further, in such a case, indescription of connection of a transistor, a source region and a sourceelectrode may be collectively referred to as a source electrode, and adrain region and a drain electrode may be collectively referred to as adrain electrode. That is, in this specification and the like, the term“source electrode” may include a source region.

An electrode 126 is connected to part of the metal compound region 124of the transistor 160. Here, the electrode 126 functions as a sourceelectrode or a drain electrode of the transistor 160. Further, thesubstrate 100 is provided with an element isolation insulating layer 106which surrounds the transistor 160. An insulating layer 128 is providedin contact with the transistor 160. Note that in order to realize highintegration, it is preferable that the transistor 160 do not have asidewall insulating layer as illustrated in FIG. 1. On the other hand,in the case where characteristics of the transistor 160 are emphasized,a sidewall insulating layer may be provided on a side surface of thegate electrode 110 and the impurity regions 120 may include an impurityregion having a different impurity concentration in a region overlappingwith the sidewall insulating layer.

The transistor 162 includes an oxide semiconductor layer 144 providedover the insulating layer 128 and the like; a source or drain electrode142 a and a source or drain electrode 142 b which are electricallyconnected to the oxide semiconductor layer 144; a gate insulating layer146 covering the oxide semiconductor layer 144, the source or drainelectrode 142 a, and the source or drain electrode 142 b; and a gateelectrode 148 a provided over the gate insulating layer 146 so as tooverlap with the oxide semiconductor layer 144. Note that the transistor170 and the transistor 172 in the second stack 210 b can each have astructure similar to that of the transistor 162.

Here, an oxide semiconductor layer used for a transistor, such as theoxide semiconductor layer 144, is preferably an oxide semiconductorlayer which is highly purified by sufficiently removing an impurity suchas hydrogen and by sufficiently supplying oxygen. Specifically, thehydrogen concentration of the oxide semiconductor layer is lower than orequal to 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, further preferably lower than or equal to 5×10¹⁷ atoms/cm³,for example. Note that the above hydrogen concentration of the oxidesemiconductor layer is measured by secondary ion mass spectrometry(SIMS). In the oxide semiconductor layer in which the hydrogenconcentration is sufficiently reduced and defect levels in the energygap due to oxygen deficiency are reduced by sufficiently supplyingoxygen, the carrier concentration is lower than 1×10¹²/cm³, preferablylower than 1×10¹¹/cm³, further preferably lower than 1.45×10¹⁰/cm³. Forexample, the off-state current (here, current per micrometer (μm) ofchannel width) at room temperature (25° C.) is lower than or equal to100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A), preferably lower than or equalto 10 zA. Thus, by using an i-type (intrinsic) or substantially i-typeoxide semiconductor, a transistor (such as the transistor 162, thetransistor 170, or the transistor 172) which has extremely excellentoff-state current characteristics can be obtained.

Note that although the transistor 162, the transistor 170, thetransistor 172, and the like each include an oxide semiconductor layerprocessed into an island shape in order to suppress leakage currentbetween elements which is caused owing to miniaturization, an oxidesemiconductor layer which is not processed into an island shape may beemployed. When an oxide semiconductor layer is not processed into anisland shape, contamination of the oxide semiconductor layer due toetching in the processing can be prevented.

The capacitor 164 includes the source or drain electrode 142 a, the gateinsulating layer 146, and a conductive layer 148 b. That is, the sourceor drain electrode 142 a functions as one electrode of the capacitor164, and the conductive layer 148 b functions as the other electrode ofthe capacitor 164. With such a structure, sufficient capacitance can besecured. Further, insulation between the source or drain electrode 142 aand the conductive layer 148 b can be sufficiently secured by stackingthe oxide semiconductor layer 144 and the gate insulating layer 146. Inthe case where a capacitor is not needed, the capacitor 164 may beomitted. The structure of the capacitor 174 is similar to that of thecapacitor 164.

Note that in the transistor 162 and the capacitor 164, end portions ofthe source or drain electrode 142 a and the source or drain electrode142 b are preferably tapered. When the end portions of the source ordrain electrode 142 a and the source or drain electrode 142 b aretapered, coverage with the gate insulating layer 146 can be improved anddisconnection can be prevented. Here, a taper angle is, for example,greater than or equal to 30° and less than or equal to 60°. Note thatthe taper angle refers to an inclination angle formed by a side surfaceand a bottom surface of a layer having a tapered shape (e.g., the sourceor drain electrode 142 a) when seen from the direction perpendicular toa cross section (a plane perpendicular to a surface of a substrate) ofthe layer. The same applies to the transistor 170, the transistor 172,and the capacitor 174.

An insulating layer 150 and an insulating layer 152 are provided overthe transistor 162 and the capacitor 164. An electrode 154 is providedin an opening formed in the gate insulating layer 146, the insulatinglayer 150, the insulating layer 152, and the like, and a wiring 156 isformed over the insulating layer 152 to be connected to the electrode154. Note that although the metal compound region 124, the source ordrain electrode 142 b, and the wiring 156 are connected to one anotherthrough the electrode 126 and the electrode 154 in FIG. 1, oneembodiment of the present invention is not limited to this. For example,the source or drain electrode 142 b may be directly connected to themetal compound region 124. Alternatively, the wiring 156 may be directlyconnected to the source or drain electrode 142 b.

Note that in FIG. 1, the electrode 126 connecting the metal compoundregion 124 to the source or drain electrode 142 b and the electrode 154connecting the source or drain electrode 142 b to the wiring 156 overlapwith each other. In other words, a region in which the electrode 126functioning as a source electrode or a drain electrode of the transistor160 is in contact with the source or drain electrode 142 b of thetransistor 162 overlaps with a region in which the source or drainelectrode 142 b of the transistor 162 is in contact with the electrode154. With such a planar layout, an increase in the element area due tocontact regions of the electrodes or wirings can be suppressed. In otherwords, the integration degree of the semiconductor device can beincreased.

In this embodiment, the transistor 160 in the first stack 210 a isprovided so as to overlap at least partly with the transistor 162 andthe capacitor 164. For example, the conductive layer 148 b of thecapacitor 164 is provided to overlap at least partly with the source ordrain electrode 142 a of the transistor 162 and the gate electrode 110of the transistor 160. With such a planar layout, the integration degreeof a memory cell can be increased. For example, given that the minimumfeature size is F, the area occupied by a memory cell can be 15F² to25F².

Similarly, in the second stack 210 b, the transistor 170 is provided soas to overlap at least partly with the transistor 172 and the capacitor174. For example, an electrode (corresponding to the conductive layer148 b of the capacitor 164) of the capacitor 174 is provided to overlapat least partly with a source or drain electrode (corresponding to thesource or drain electrode 142 a of the transistor 162) of the transistor172 and a gate electrode (corresponding to the gate electrode 110 of thetransistor 160) of the transistor 170. Thus, the integrated first stack210 a and the integrated second stack 210 b are stacked with aninsulating layer 158 positioned therebetween, whereby higher integrationof the semiconductor device can be achieved.

Note that FIG. 1 illustrates the example in which two layers, the firststack 210 a and the second stack 210 b are stacked; however, oneembodiment of the present invention is not limited to this, and astacked structure including three or more layers can be employed. Inthat case, a third stack can have a structure similar to that of thesecond stack 210 b. With such a stacked structure, still higherintegration of the semiconductor device can be achieved.

<Circuit Configuration of Semiconductor Device>

Next, a circuit configuration and operation of the semiconductor deviceillustrated in FIG. 1 will be described with reference to FIGS. 2A-1,2A-2, 2B, and 2C. Note that in some circuit diagrams, “OS” is writtenbeside a transistor in order to indicate that the transistor includes anoxide semiconductor.

<Basic Configuration>

In a semiconductor device illustrated in FIG. 2A-1, a first wiring (a1st Line) is electrically connected to a source electrode (or a drainelectrode) of the transistor 160. A second wiring (a 2nd Line) iselectrically connected to the drain electrode (or the source electrode)of the transistor 160. In addition, a third wiring (a 3rd Line) iselectrically connected to a source electrode (or a drain electrode) ofthe transistor 162, and a fourth wiring (a 4th Line) is electricallyconnected to a gate electrode of the transistor 162. Further, a gateelectrode of the transistor 160 and the drain electrode (or the sourceelectrode) of the transistor 162 are electrically connected to oneelectrode of the capacitor 164, and a fifth wiring (a 5th Line) iselectrically connected to the other electrode of the capacitor 164. Notethat the circuit configuration illustrated in FIG. 2A-1 corresponds to aconfiguration of a circuit included in the first stack 210 a illustratedin FIG. 1, and the circuit illustrated in FIG. 2A-1 functions as amemory cell.

Here, the above transistor including an oxide semiconductor is used asthe transistor 162, for example. A transistor including an oxidesemiconductor has a characteristic of extremely low off-state current.For that reason, a potential of the gate electrode of the transistor 160can be held for an extremely long time by turning off the transistor162. Provision of the capacitor 164 facilitates holding of chargesupplied to the gate electrode of the transistor 160 and reading ofstored data.

Although there is no particular limitation on the transistor 160, atransistor including a semiconductor material other than an oxidesemiconductor is used as the transistor 160, for example. In terms ofincreasing the speed of data reading, it is preferable to use, forexample, a transistor with high switching speed such as a transistorincluding single crystal silicon.

In a semiconductor device illustrated in FIG. 2B, a first wiring (a 1stLine) is electrically connected to a source electrode (or a drainelectrode) of the transistor 170. A second wiring (a 2nd Line) iselectrically connected to the drain electrode (or the source electrode)of the transistor 170. In addition, a third wiring (a 3rd Line) iselectrically connected to a source electrode (or a drain electrode) ofthe transistor 172, and a fourth wiring (a 4th Line) is electricallyconnected to a gate electrode of the transistor 172. Further, a gateelectrode of the transistor 170 and the drain electrode (or the sourceelectrode) of the transistor 172 are electrically connected to oneelectrode of the capacitor 174, and a fifth wiring (a 5th Line) iselectrically connected to the other electrode of the capacitor 174. Notethat the circuit configuration illustrated in FIG. 2B corresponds to aconfiguration of a circuit included in the second stack 210 billustrated in FIG. 1.

Here, the above transistor including an oxide semiconductor is used asthe transistor 170 and the transistor 172. The transistor including anoxide semiconductor has a characteristic of extremely low off-statecurrent. For that reason, a potential of the gate electrode of thetransistor 170 can be held for an extremely long time by turning off thetransistor 172. Provision of the capacitor 174 facilitates holding ofcharge supplied to the gate electrode of the transistor 170 and readingof stored data. Note that the transistors 170 and 172 including an oxidesemiconductor each have a channel length (L) of greater than or equal to10 nm and less than or equal to 1000 nm; therefore, power consumption islow and operation speed is sufficiently high.

As illustrated in FIG. 2C, the capacitor 164 can be omitted in FIG.2A-1. In a similar manner, the capacitor 174 can be omitted in FIG. 2B.

The semiconductor device illustrated in FIG. 2A-1 utilizes acharacteristic in which the potential of the gate electrode of thetransistor 160 can be held, whereby writing, holding, and reading ofdata can be performed as follows. In the semiconductor deviceillustrated in FIG. 2B, writing, holding, and reading of data can beperformed in a manner similar to that in the semiconductor deviceillustrated in FIG. 2A-1; therefore, detailed description is omitted.

Firstly, writing and holding of data will be described with reference toFIG. 2A-1. First, the potential of the fourth wiring is set to apotential at which the transistor 162 is turned on, so that thetransistor 162 is turned on. Accordingly, the potential of the thirdwiring is supplied to the gate electrode of the transistor 160 and thecapacitor 164. That is, predetermined charge is supplied to the gateelectrode of the transistor 160 (writing). Here, one of charges forsupply of two different potentials (hereinafter, a charge for supply ofa low potential is referred to as a charge Q_(L) and a charge for supplyof a high potential is referred to as a charge Q_(H)) is supplied. Notethat charges for supply of three or more different potentials may beemployed to improve storage capacity. After that, the potential of thefourth wiring is set to a potential at which the transistor 162 isturned off, so that the transistor 162 is turned off. Thus, the chargesupplied to the gate electrode of the transistor 160 is held (holding).

Since the off-state current of the transistor 162 is extremely low, thecharge of the gate electrode of the transistor 160 is held for a longtime.

Secondly, reading of data will be described. By supplying an appropriatepotential (a reading potential) to the fifth wiring while apredetermined potential (a constant potential) is supplied to the firstwiring, the potential of the second wiring varies depending on theamount of charge held in the gate electrode of the transistor 160. Thisis because in general, when the transistor 160 is an n-channeltransistor, an apparent threshold voltage V_(th) _(_) _(H) in the casewhere Q_(H) is supplied to the gate electrode of the transistor 160 islower than an apparent threshold voltage V_(th) _(_) _(L) in the casewhere Q_(L) is supplied to the gate electrode of the transistor 160.Here, an apparent threshold voltage refers to the potential of the fifthwiring, which is needed to turn on the transistor 160. Thus, thepotential of the fifth wiring is set to a potential V₀ intermediatebetween V_(th) _(_) _(H) and V_(th) _(_) _(L), whereby charge suppliedto the gate electrode of the transistor 160 can be determined. Forexample, in the case where Q_(H) is supplied in writing, when thepotential of the fifth wiring is V₀ (>V_(th) _(_) _(H)), the transistor160 is turned on. In the case where Q_(L) is supplied in writing, evenwhen the potential of the fifth wiring is V₀ (<V_(th) _(_) _(L)), thetransistor 160 remains in an off state. Therefore, the stored data canbe read by measuring the potential of the second wiring.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells is needed to be read. Thus, in the casewhere data of predetermined memory cells is read and data of the othermemory cells is not read, a potential which allows the transistor 160 tobe turned off regardless of a state of the gate electrode, that is, apotential lower than V_(th) _(_) _(H) may be supplied to fifth wiringsof the memory cells whose data is not to be read. Alternatively, apotential which allows the transistor 160 to be turned on regardless ofa state of the gate electrode, that is, a potential higher than V_(th)_(_) _(L) may be supplied to the fifth wirings.

Thirdly, rewriting of data will be described. Rewriting of data isperformed in a manner similar to that of the writing and holding ofdata. That is, the potential of the fourth wiring is set to a potentialat which the transistor 162 is turned on, so that the transistor 162 isturned on. Accordingly, the potential of the third wiring (a potentialrelating to new data) is supplied to the gate electrode of thetransistor 160 and the capacitor 164. After that, the potential of thefourth wiring is set to a potential at which the transistor 162 isturned off, so that the transistor 162 is turned off. Thus, charge fornew data is supplied to the gate electrode of the transistor 160.

In the semiconductor device according to one embodiment of the presentinvention, data can be directly rewritten by another write operation ofdata as described above. Therefore, extraction of charge from a floatinggate with the use of high voltage, which is necessary in a flash memoryor the like, is not needed and a decrease in operation speed due toerasing operation can be suppressed. In other words, high-speedoperation of the semiconductor device can be realized.

Note that the drain electrode (or the source electrode) of thetransistor 162 is electrically connected to the gate electrode of thetransistor 160, thereby having a function similar to that of a floatinggate of a floating-gate transistor which is used as a nonvolatile memoryelement. Therefore, a portion in the drawing where the drain electrode(or the source electrode) of the transistor 162 is electricallyconnected to the gate electrode of the transistor 160 is called afloating gate portion FG in some cases. When the transistor 162 is off,the floating gate portion FG can be regarded as being embedded in aninsulator and charge is held in the floating gate portion FG. Theoff-state current of the transistor 162 including an oxide semiconductoris lower than or equal to one hundred thousandth of the off-statecurrent of a transistor including a silicon semiconductor or the like;thus, loss of the charge accumulated in the floating gate portion FG dueto leakage current of the transistor 162 is negligible. That is, withthe transistor 162 including an oxide semiconductor, a nonvolatilememory device which can hold data without being supplied with power canbe realized.

For example, when the off-state current of the transistor 162 is 10 zA(1 zA (zeptoampere) is 1×10⁻²¹ A) or less at room temperature (25° C.)and the capacitance of the capacitor 164 is approximately 10 fF, datacan be held for 10⁴ seconds or more. Needless to say, the holding timedepends on transistor characteristics and the capacitance.

Further, in that case, the problem of deterioration of a gate insulatinglayer (a tunnel insulating layer), which is pointed out in aconventional floating-gate transistor, does not arise. That is, theproblem of deterioration of a gate insulating layer due to injection ofelectrons into a floating gate, which has been regarded as a problem,can be solved. This means that there is no limit on the number of writeoperations in principle. Furthermore, high voltage needed for writing orerasing in a conventional floating-gate transistor is not necessary.

Components such as transistors in the semiconductor device illustratedin FIG. 2A-1 can be regarded as including resistors and capacitors asillustrated in FIG. 2A-2. That is, in FIG. 2A-2, the transistor 160 andthe capacitor 164 are each regarded as including a resistor and acapacitor. R1 and C1 denote the resistance and the capacitance of thecapacitor 164, respectively. The resistance R1 corresponds to theresistance of an insulating layer included in the capacitor 164. R2 andC2 denote the resistance and the capacitance of the transistor 160,respectively. The resistance R2 corresponds to the resistance of a gateinsulating layer at the time when the transistor 160 is on. Thecapacitance C2 corresponds to so-called gate capacitance (capacitanceformed between the gate electrode and the source electrode or the drainelectrode and capacitance formed between the gate electrode and thechannel formation region).

A charge holding period (also referred to as a data holding period) isdetermined mainly by the off-state current of the transistor 162 underthe conditions where gate leakage of the transistor 162 is sufficientlysmall and the relations R1≧ROS and R2≧ROS are satisfied, where theresistance (also referred to as effective resistance) between the sourceelectrode and the drain electrode in the case where the transistor 162is off is ROS.

On the other hand, when the conditions are not met, it is difficult tosecure a sufficient holding period even if the off-state current of thetransistor 162 is low enough. This is because leakage current other thanthe off-state current of the transistor 162 (e.g., leakage currentgenerated between the source electrode and the gate electrode) is high.Thus, it can be said that the above relations are preferably satisfiedin the semiconductor device disclosed in this embodiment.

It is preferable that the relation C1≧C2 be satisfied. This is becausewhen C1 is large, the potential of the fifth wiring can be supplied tothe floating gate portion FG efficiently at the time of controlling thepotential of the floating gate portion FG by the fifth wiring, and adifference between potentials (e.g., the reading potential and anon-reading potential) supplied to the fifth wiring can be reduced.

When the above relation is satisfied, a more favorable semiconductordevice can be realized. Note that R1 and R2 are controlled by the gateinsulating layer of the transistor 160 and the insulating layer of thecapacitor 164. The same applies to C1 and C2. Therefore, the material,thickness, and the like of the gate insulating layer are preferably setas appropriate to satisfy the above relation.

In the semiconductor device described in this embodiment, the floatinggate portion FG has a function similar to that of a floating gate of afloating-gate transistor of a flash memory or the like, but the floatinggate portion FG of this embodiment has a feature which is essentiallydifferent from that of the floating gate of the flash memory or thelike. In the case of a flash memory, since voltage applied to a controlgate is high, it is necessary to keep a proper distance between cells inorder to prevent the potential from affecting a floating gate of theadjacent cell. This is one of factors inhibiting higher integration ofthe semiconductor device. The factor is attributed to a basic principleof a flash memory, in which tunneling current is generated by applying ahigh electric field.

In contrast, the semiconductor device according to this embodiment isoperated by switching of a transistor including an oxide semiconductorand does not use the above-described principle of charge injection bytunneling current. That is, a high electric field for charge injectionis not necessary unlike a flash memory. Accordingly, it is not necessaryto consider an influence of a high electric field from a control gate onan adjacent cell, which facilitates higher integration.

In addition, there is another advantage over a flash memory in that ahigh electric field and a large peripheral circuit (such as a boostercircuit) are unnecessary. For example, the highest voltage applied tothe memory cell according to this embodiment (the difference between thehighest potential and the lowest potential applied to terminals of thememory cell at the same time) can be 5 V or lower, preferably 3 V orlower in each memory cell in the case where two-level (one-bit) data iswritten.

In the case where the dielectric constant H of the insulating layerincluded in the capacitor 164 is different from the dielectric constant∈r2 of the insulating layer included in the transistor 160, it is easyto satisfy C1≧C2 while 2·S2≧S1 (preferably, S2≧S1) is satisfied, whereS1 is the area of the insulating layer included in the capacitor 164 andS2 is the area of the insulating layer forming a gate capacitor of thetransistor 160. Specifically, for example, a film formed using a high-kmaterial such as hafnium oxide or a stack of a film formed using ahigh-k material such as hafnium oxide and a film formed using an oxidesemiconductor is used for the insulating layer included in the capacitor164 so that ∈r1 can be 10 or more, preferably 15 or more; silicon oxideis used for the insulating layer forming the gate capacitor so that ∈r2can be 3 to 4.

A combination of such structures enables still higher integration of thesemiconductor device according to one embodiment of the presentinvention.

Note that in addition to the increase in the integration degree, amultilevel technique can be employed in order to increase the storagecapacity of the semiconductor device. For example, data of three or morelevels is written into one memory cell, whereby the storage capacity canbe increased as compared to the case where data of two levels iswritten. The multilevel technique can be achieved by, for example,giving charge Q to a gate electrode of a first transistor, in additionto the charge Q_(L) for supply of a low potential and the charge Q_(H)for supply of a high potential, which are described above. In this case,enough storage capacity can be secured even when a circuit configurationin which F² is not sufficiently small is employed.

Note that an n-channel transistor in which electrons are majoritycarriers is used in the above description; needless to say, a p-channeltransistor in which holes are majority carriers can be used instead ofthe n-channel transistor.

As described above, the semiconductor device according to thisembodiment is suitable for increasing the integration degree. Note thataccording to one embodiment of the present invention, a wiring is sharedand the contact region is reduced; thus, a semiconductor device whoseintegration degree is further increased can be provided.

<Application Example>

An application example of the above semiconductor device will bedescribed with reference to FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, FIG.8, FIG. 9, FIG. 10, FIG. 11, FIG. 12, FIG. 13, FIG. 14, FIG. 15, FIGS.16A to 16D, FIGS. 17A to 17D, FIGS. 18A to 18D, and FIGS. 19A and 19B.

FIG. 3 is an example of a block diagram of a semiconductor device. Thesemiconductor device illustrated in FIG. 3 includes a memory cell array201, a first driver circuit 202, and a second driver circuit 203.

First, the memory cell array 201 is described. The memory cell array 201includes memory cell arrays 211 a to 211 c which are stacked.

The memory cell array 211 a includes n (n is an integer of 2 or more)bit lines BL, m (m is an integer of 2 or more) signal lines S, m wordlines WL, k (k is a natural number of n or less or a natural number of mor less) source lines SL, and a region in which memory cells 212 a arearranged in matrix of m (rows) (in a vertical direction)×n (columns) (ina horizontal direction). Here, the memory cell 212 a preferably has theconfiguration illustrated in FIG. 2A-1. In addition, the signal lines Sconnected to the memory cell array 211 a are denoted by signal lines S(1,1) to S (m,1), and the word lines WL connected to the memory cellarray 211 a are denoted by word lines WL (1,1) to WL (m,1). The bitlines BL connected to the memory cell array 211 a are denoted by bitlines BL (1,1) to BL (n,1).

Each of the memory cells 212 a includes a first transistor, a secondtransistor, and a first capacitor. Here, the memory cell 212 a has astructure corresponding to that of the first stack 210 a illustrated inFIG. 1. In the memory cell 212 a, the first transistor, the secondtransistor, and the first capacitor correspond to the transistor 160,the transistor 162, and the capacitor 164 in the configurationillustrated in FIG. 2A-1, respectively. In each of the memory cells 212a, a gate electrode of the first transistor, a drain electrode (or asource electrode) of the second transistor, and one electrode of thefirst capacitor are electrically connected to one another, and thesource line SL and a source electrode of the first transistor areelectrically connected to each other. Further, the bit line BL, thesource electrode (or the drain electrode) of the second transistor, anda drain electrode of the first transistor are electrically connected toone another. The word line WL and the other electrode of the firstcapacitor are electrically connected to each other. The signal line Sand a gate electrode of the second transistor are electrically connectedto each other. In other words, the source line SL corresponds to thefirst wiring (the 1st Line) in the configuration illustrated in FIG.2A-1, the bit line BL corresponds to the second wiring (the 2nd Line)and the third wiring (the 3rd Line), the signal line S corresponds tothe fourth wiring (the 4th Line), and the word line WL corresponds tothe fifth wiring (the 5th Line).

The memory cell 212 a including the first semiconductor material and thesecond semiconductor material is used in the memory cell array 211 a,whereby the speed of read operation can be increased with a sufficientholding period secured.

The memory cell array 211 b includes n (n is an integer of 2 or more)bit lines BL, m (m is an integer of 2 or more) signal lines S, m wordlines WL, k (k is a natural number of n or less or a natural number of mor less) source lines SL, and a region in which memory cells 212 b arearranged in matrix of m (rows) (in a vertical direction)×n (columns) (ina horizontal direction). Here, the memory cell 212 b preferably has theconfiguration illustrated in FIG. 2B. In addition, the signal lines Sconnected to the memory cell array 211 b are denoted by signal lines S(1,2) to S (m,2), and the word lines WL connected to the memory cellarray 211 b are denoted by word lines WL (1,2) to WL (m,2). The bitlines BL connected to the memory cell array 211 b are denoted by bitlines BL (1,2) to BL (n,2).

Each of the memory cells 212 b includes a third transistor, a fourthtransistor, and a second capacitor. Here, the memory cell 212 b has astructure corresponding to that of the second stack 210 b illustrated inFIG. 1. In the memory cell 212 b, the third transistor, the fourthtransistor, and the second capacitor correspond to the transistor 170,the transistor 172, and the capacitor 174 in the configurationillustrated in FIG. 2B, respectively. In each of the memory cells 212 b,a gate electrode of the third transistor, a drain electrode (or a sourceelectrode) of the fourth transistor, and one electrode of the secondcapacitor are electrically connected to one another, and the source lineSL and a source electrode of the third transistor are electricallyconnected to each other. Further, the bit line BL, the source electrode(or the drain electrode) of the fourth transistor, and a drain electrodeof the third transistor are electrically connected to one another. Theword line WL and the other electrode of the second capacitor areelectrically connected to each other. The signal line S and a gateelectrode of the fourth transistor are electrically connected to eachother. In other words, the source line SL corresponds to the firstwiring (the 1st Line) in the configuration illustrated in FIG. 2B, thebit line BL corresponds to the second wiring (the 2nd Line) and thethird wiring (the 3rd Line), the signal line S corresponds to the fourthwiring (the 4th Line), and the word line WL corresponds to the fifthwiring (the 5th Line).

Note that the memory cell array 211 c can have a configuration similarto that of the memory cell array 211 b and thus is not described indetail. That is, the memory cell array 211 c includes a plurality ofmemory cells 212 c. In addition, the signal lines S connected to thememory cell array 211 c are denoted by signal lines S (1,3) to S (m,3),and the word lines WL connected to the memory cell array 211 c aredenoted by word lines WL (1,3) to WL (m,3). The bit lines BL connectedto the memory cell array 211 c are denoted by bit lines BL (1,3) to BL(n,3).

The memory cell 212 b and the memory cell 212 c which include the secondsemiconductor material are used in the memory cell array 211 b and thememory cell array 211 c, whereby a sufficient data holding period can besecured without making the manufacturing process complicated.

Further, the memory cell arrays 211 a to 211 c are stacked, wherebyhigher integration of the semiconductor device can be achieved.

Note that FIG. 3 illustrates the case where the memory cell arrays 211 ato 211 c are not connected to one another; however, one embodiment ofthe present invention is not limited to this. For example, the sourceline SL included in the memory cell array 211 a may be electricallyconnected to the source line SL included in the memory cell array 211 bso that the memory cell 212 a is electrically connected to the memorycell 212 b. Consequently, the number of source lines SL can be reduced.Alternatively, the source line SL connected to the memory cell 212 a maybe electrically connected the source line SL connected to the memorycell 212 b, so that the memory cell 212 a can be electrically connectedto the memory cell 212 b.

In the memory cell arrays 211 a to 211 c of the semiconductor deviceillustrated in FIG. 3, the memory cells are arranged in matrix of m(rows) (in a vertical direction)×n (columns) (in a horizontaldirection); however, one embodiment of the present invention is notlimited to this. The memory cell arrays 211 a to 211 c do notnecessarily have the same memory cell configuration, and can havedifferent memory cell configurations.

The first driver circuit 202 and the second driver circuit 203 eachinclude a plurality of circuits. The first driver circuit 202 includes aselector 221, a circuit 222 including a buffer or the like, and a rowdecoder 223. The second driver circuit 203 includes a selector 231, acircuit group 232, and a column decoder 233. The circuit group 232includes a writing circuit group 234, a reading circuit group 235, and aregister group 236.

Here, FIG. 4 is a simplified block diagram illustrating an example of astacked structure of the semiconductor device illustrated in FIG. 3. Inthe semiconductor device illustrated in FIG. 4, the memory cell array201 includes three stacks, and the first driver circuit 202 and thesecond driver circuit 203 each include a single stack. The memory cellarray 211 a is provided in the first stack 210 a, the memory cell array211 b is provided in the second stack 210 b, and the memory cell array211 c is provided in a stack over the second stack 210 b. The firstdriver circuit 202 and the second driver circuit 203 are provided in thefirst stack 210 a.

FIG. 5 is an example of a circuit diagram of the selector 231 in thesemiconductor device illustrated in FIG. 3 and FIG. 4. Here, the casewhere the selector 231 is provided in the first stack 210 a isdescribed. The selector 231 includes a plurality of transistors.Further, the selector 231 is connected to the memory cell array 211 athrough BL (1,1) to BL (n,1). In a similar manner, the selector 231 isconnected to the memory cell array 211 b through BL (1,2) to BL (n,2)and connected to the memory cell array 211 c through BL (1,3) to BL(n,3). The selector 231 electrically connects the bit lines BL toterminals in the circuit group 232 in accordance with layer selectionsignals LAY1, LAY2, and LAY3. When the signal LAY1 is active, BL (1,1)to BL (n,1) are electrically connected to the terminals in the circuitgroup 232. When the signal LAY2 is active, BL (1,2) to BL (n,2) areelectrically connected to the terminals in the circuit group 232. Whenthe signal LAY3 is active, BL (1,3) to BL (n,3) are electricallyconnected to the terminals in the circuit group 232.

FIG. 6 is an example of a block diagram of the circuit group 232 in thesemiconductor device illustrated in FIG. 3 and FIG. 4. Here, the casewhere the circuit group 232 is provided in the first stack 210 a isdescribed. The circuit group 232 includes the writing circuit group 234,the reading circuit group 235, and the register group 236. The writingcircuit group 234 includes a plurality of writing circuits 237. A writeenable signal WE, a writing potential Vwrite, and signals output fromthe register group 236 are input to the writing circuit group 234.Output signals from the plurality of writing circuits 237 are input tothe selector 231. The reading circuit group 235 includes readingcircuits 238. A read enable signal RE and a reading potential Vread areinput to the reading circuit group 235, whose output signals are inputto the register group 236. Reading terminals of the reading circuits 238are connected to the selector 231. Input data DIN is input to theregister group 236, and output data DOUT is output from the registergroup 236. Further, an output signal of the reading circuit group 235 isinput to the register group 236, and a signal input to the writingcircuit group 234 is output from the register group 236. The signalinput to the writing circuit group 234 may be a pair of signals havingopposite phases. Note that the numbers of writing circuits 237, readingcircuits 238, and registers 239 are each the same as the number ofcolumns of the memory cell array.

Operation of the circuit group 232 illustrated in FIG. 6 is described.The following operations are described: operation in which data iswritten into the register group 236 from the outside, operation in whichdata is read from the register group 236 to the outside, operation inwhich data is written into a memory cell from the register group 236,and operation in which data is read from a memory cell to the registergroup 236.

Data is written into the register group 236 from the outside by input ofthe signal DIN to the register group 236. Data is read from the registergroup 236 to the outside by output of data which is stored in theregister group 236 as the signal DOUT. In addition, the operation inwhich data is written into a memory cell from the register group 236 isperformed in such a manner that the writing circuit group 234 selectswriting voltage on the basis of a signal output from the register group236 and outputs the writing voltage during a period in which the writeenable signal WE is active. As a result, the writing voltage is suppliedto the bit line BL and the data is written into the memory cell. Theoperation in which data is read from a memory cell to the register group236 is performed in such a manner that, during a period in which theread enable signal RE is active, the reading circuit group 235 readsdata from a memory cell by determining a bit-line potential and outputsthe data, and the output data is stored in the register group 236.

As the reading circuit 238, for example, a reading circuit illustratedin FIG. 7 can be used. The reading circuit illustrated in FIG. 7includes a sense amplifier SA, a transistor serving as a load, and aswitch. The sense amplifier SA compares a bit-line potential and thereading potential Vread and outputs the comparison result. The readenable signal RE determines whether the reading circuit is electricallyconnected to the bit line.

As the writing circuit 237, for example, a writing circuit illustratedin FIG. 8 can be used. The writing circuit illustrated in FIG. 8includes three switches. In accordance with a pair of signals havingopposite phases, either one of potentials Vwrite and GND is selected.The write enable signal WE determines whether the selected potential issupplied or not.

FIG. 9 is an example of a circuit diagram of the selector 221 in thesemiconductor device illustrated in FIG. 3 and FIG. 4. Here, the casewhere the selector 221 is provided in the first stack 210 a isdescribed.

The selector 221 includes a plurality of transistors. Further, theselector 221 is connected to the memory cell array 211 a through WL(1,1) to WL (m,1) and S (1,1) to S (m,1). In a similar manner, theselector 221 is connected to the memory cell array 211 b through WL(1,2) to WL (m,2) and S (1,2) to S (m,2) and connected to the memorycell array 211 c through WL (1,3) to WL (m,3) and S (1,3) to S (m,3).The selector 221 electrically connects the word lines WL and the signallines S to terminals in the circuit 222 in accordance with the layerselection signals LAY1, LAY2, and LAY3. When the signal LAY1 is active,WL (1,1) to WL (m,1) and S (1,1) to S (m,1) are electrically connectedto the terminals in the circuit 222. When the signal LAY2 is active, WL(1,2) to WL (m,2) and S (1,2) to S (m,2) are electrically connected tothe terminals in the circuit 222. When the signal LAY3 is active, WL(1,3) to WL (m,3) and S (1,3) to S (m,3) are electrically connected tothe terminals in the circuit 222.

By forming circuits provided in the first driver circuit 202 and thesecond driver circuit 203 with the use of the first semiconductormaterial, the first driver circuit 202 and the second driver circuit 203can operate at high speed.

FIG. 10 is a simplified block diagram illustrating another example of astacked structure of the semiconductor device illustrated in FIG. 3. Inthe semiconductor device illustrated in FIG. 10, the memory cell array201 includes three stacks, selectors 221 a, 221 b, and 221 c included inthe first driver circuit 202 are three stacks, selectors 231 a, 231 b,and 231 c included in the second driver circuit 203 are three stacks,and the other circuits in the first driver circuit 202 and the seconddriver circuit 203 each include a single stack. The memory cell array211 a, the selector 221 a, and the selector 231 a are provided in thefirst stack 210 a. The memory cell array 211 b, the selector 221 b, andthe selector 231 b are provided in the second stack 210 b. The memorycell array 211 c, the selector 221 c, and the selector 231 c areprovided in a third stack 210 c.

FIG. 11 is an example of a circuit diagram of the selectors 231 a, 231b, and 231 c in the second driver circuit 203 of the semiconductordevice illustrated in FIG. 10. The selectors illustrated in FIG. 11 havea circuit configuration similar to that of the selector illustrated inFIG. 5. A stacked structure of the selector is different between FIG. 5and FIG. 11. The transistors included in the selector are provided inthe first stack 210 a in FIG. 5, whereas transistors included in theselectors are provided in three layers in FIG. 11. For example, in thesecond driver circuit 203, the selector 231 a is formed in the samelayer as the memory cell array 211 a, the selector 231 b is formed inthe same layer as the memory cell array 211 b, and the selector 231 c isformed in the same layer as the memory cell array 211 c. In other words,in the second driver circuit 203, the selector 231 a includes the firstsemiconductor material, and the selector 231 b and the selector 231 cinclude the second semiconductor material.

The circuit group 232 of the semiconductor device illustrated in FIG. 10may have the same circuit configuration and stacked structure as thecircuit group 232 of the semiconductor device illustrated in FIG. 4. Theexample of the block diagram of FIG. 6 can be referred to for thedetails.

FIG. 12 is an example of a circuit diagram of the selectors 221 a, 221b, and 221 c in the first driver circuit 202 of the semiconductor deviceillustrated in FIG. 10. The selectors illustrated in FIG. 12 have acircuit configuration similar to that of the selector illustrated inFIG. 9. A stacked structure of the selector is different between FIG. 9and FIG. 12. The transistors included in the selector are provided inthe first stack 210 a in FIG. 9, whereas transistors included in theselectors are provided in three layers in FIG. 12. For example, in thefirst driver circuit 202, the selector 221 a is formed in the same layeras the memory cell array 211 a, the selector 221 b is formed in the samelayer as the memory cell array 211 b, and the selector 221 c is formedin the same layer as the memory cell array 211 c. In other words, in thefirst driver circuit 202, the selector 221 a includes the firstsemiconductor material, and the selector 221 b and the selector 221 cinclude the second semiconductor material.

With such a configuration, the area occupied by a driver circuit can bereduced and the memory density can be increased. Further, an increase inthe area of a memory cell array leads to higher storage capacity.

FIG. 13 is a simplified block diagram illustrating another example of astacked structure of the semiconductor device illustrated in FIG. 3. Inthe semiconductor device illustrated in FIG. 13, the memory cell array201 includes three stacks, and the first driver circuit 202 and thesecond driver circuit 203 each partly have a stacked structure includingplural layers. The memory cell array 211 a, the selector 221 a, theselector 231 a, and a circuit group 232 a are provided in the firststack 210 a. The memory cell array 211 b, the selector 221 b, theselector 231 b, and a circuit group 232 b are provided in the secondstack 210 b. The memory cell array 211 c, the selector 221 c, and theselector 231 c are provided in the third stack 210 c.

The structure of the second driver circuit 203 is different between FIG.10 and FIG. 13. For example, the circuit group 232 in the second drivercircuit 203 has a single-layer structure in FIG. 10, whereas the circuitgroups 232 a and 232 b in the second driver circuit 203 form a two-layerstacked structure in FIG. 13.

The selectors 231 a, 231 b, and 231 c and the selectors 221 a, 221 b,and 221 c of the semiconductor device illustrated in FIG. 13 may havethe same circuit configurations and stacked structures as the selectors231 a, 231 b, and 231 c and the selectors 221 a, 221 b, and 221 c of thesemiconductor device illustrated in FIG. 10. The examples of the blockdiagrams of FIG. 11 and FIG. 12 can be referred to for the details.

FIG. 14 is an example of a circuit diagram of the circuit groups 232 aand 232 b and the selectors 231 a, 231 b, and 231 c in the second drivercircuit 203 of the semiconductor device illustrated in FIG. 13. Thecircuit groups 232 a and 232 b illustrated in FIG. 14 have a circuitconfiguration similar to that of the circuit group 232 illustrated inFIG. 6. A stacked structure is different between the circuit group 232illustrated in FIG. 6 and the circuit groups 232 a and 232 b illustratedin FIG. 14. Transistors included in the circuit group 232 are providedin the first stack 210 a in FIG. 6, whereas transistors included in thecircuit groups 232 a and 232 b are provided in two layers in FIG. 14.For example, in the second driver circuit 203, the circuit group 232 ais formed in the same layer as the memory cell array 211 a, and thecircuit group 232 b is formed in the same layer as the memory cell array211 b. In other words, in the second driver circuit 203, the circuitgroup 232 a includes the first semiconductor material, and the circuitgroup 232 b includes the second semiconductor material.

The circuit group 232 a illustrated in FIG. 14 includes the registergroup 236 and the reading circuit group 235. The circuit group 232 billustrated in FIG. 14 includes the writing circuit group 234. As areading circuit included in the reading circuit group 235, for example,the reading circuit illustrated in FIG. 7 can be used. As a writingcircuit included in the writing circuit group 234, for example, awriting circuit illustrated in FIG. 15 can be used. The writing circuitillustrated in FIG. 15 includes three transistors including the secondsemiconductor material. In accordance with a pair of signals havingopposite phases, either one of potentials Vwrite and GND is selected.The write enable signal WE determines whether the selected potential issupplied or not.

With such a configuration, the area occupied by a driver circuit can bereduced and the memory density can be increased. Further, an increase inthe area of a memory cell array leads to higher storage capacity.

In this embodiment, an example in which the memory cell array 201, thefirst driver circuit 202, or the second driver circuit 203 has athree-layer stacked structure is described; however, one embodiment ofthe present invention is not limited to this. A stacked structureincluding two layers or a stacked structure including four or morelayers can be employed.

<Method for Manufacturing Semiconductor Device>

Next, an example of a method for manufacturing the above semiconductordevice will be described. First, a method for manufacturing thetransistor 160 in the lower portion in the first stack 210 a will bedescribed below with reference to FIGS. 16A to 16D and FIGS. 17A to 17D;then, a method for manufacturing the transistor 162 and the capacitor164 in the upper portion will be described with reference to FIGS. 18Ato 18D and FIGS. 19A and 19B. Note that a method for manufacturing thetransistor 170, the transistor 172, and the capacitor 174 in the secondstack 210 b is similar to the method for manufacturing the transistor162 and the capacitor 164, and thus is not described in detail.

<Method for Manufacturing Transistor in Lower Portion>

A method for manufacturing the transistor 160 in the lower portion willbe described with reference to FIGS. 16A to 16D and FIGS. 17A to 17D.

First, the substrate 100 including a semiconductor material is prepared.As the substrate including a semiconductor material, a single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, silicon carbide, or the like; a compound semiconductorsubstrate of silicon germanium or the like; an SOI substrate; or thelike can be used. Here, an example of using a single crystal siliconsubstrate as the substrate 100 including a semiconductor material isdescribed. Note that in general, the term “SOI substrate” means asubstrate where a silicon semiconductor layer is provided on aninsulating surface. In this specification and the like, the term “SOIsubstrate” also includes a substrate where a semiconductor layer formedusing a material other than silicon is provided over an insulatingsurface. That is, a semiconductor layer included in the “SOI substrate”is not limited to a silicon-based semiconductor layer. Moreover, the SOIsubstrate can be a substrate having a structure in which a semiconductorlayer is provided over an insulating substrate such as a glass substratewith an insulating layer positioned therebetween.

It is preferable that a single crystal semiconductor substrate such as asilicon wafer be used as the substrate 100 including a semiconductormaterial because the speed of read operation of the semiconductor devicecan be increased.

A protective layer 102 serving as a mask for forming an elementisolation insulating layer is formed over the substrate 100 (see FIG.16A). As the protective layer 102, for example, an insulating layerformed using silicon oxide, silicon nitride, silicon oxynitride, or thelike can be used. Note that before or after this step, an impurityelement imparting n-type conductivity or an impurity element impartingp-type conductivity may be added to the substrate 100 in order tocontrol the threshold voltage of the transistor. In the case wheresilicon is used as the semiconductor, phosphorus, arsenic, or the likecan be used as the impurity element imparting n-type conductivity, forexample. Boron, aluminum, gallium, or the like can be used as theimpurity element imparting p-type conductivity, for example.

Next, part of the substrate 100 in a region that is not covered with theprotective layer 102 (in an exposed region) is removed by etching withthe use of the protective layer 102 as a mask. Thus, a semiconductorregion 104 isolated from the other semiconductor regions is formed (seeFIG. 16B). As the etching, dry etching is preferably performed, but wetetching may be performed. An etching gas or an etchant can be selectedas appropriate in accordance with a material to be etched.

Next, an insulating layer is formed so as to cover the semiconductorregion 104 and the insulating layer in a region overlapping with thesemiconductor region 104 is selectively removed, so that the elementisolation insulating layer 106 is formed (see FIG. 16C). The insulatinglayer is formed using silicon oxide, silicon nitride, siliconoxynitride, or the like. As a method for removing the insulating layer,any of etching treatment, polishing treatment such as chemicalmechanical polishing (CMP), and the like may be employed. Note that theprotective layer 102 is removed after the formation of the semiconductorregion 104 or after the formation of the element isolation insulatinglayer 106.

Here, the CMP treatment is a method for planarizing a surface of anobject to be processed by a combination of chemical and mechanicalactions with the use of the surface as a reference. Specifically, theCMP treatment is a method in which a polishing cloth is attached to apolishing stage, the polishing stage and the object to be processed areeach rotated or swung while a slurry (an abrasive) is supplied betweenthe object to be processed and the polishing cloth, and the surface ofthe object to be processed is polished by chemical reaction between theslurry and the surface of the object to be processed and by an action ofmechanical polishing of the object to be processed with the polishingcloth.

Note that as a method for forming the element isolation insulating layer106, a method in which an insulating region is formed by introduction ofoxygen, or the like can be used instead of the method in which theinsulating layer is selectively removed.

Next, an insulating layer is formed over a surface of the semiconductorregion 104, and a layer including a conductive material is formed overthe insulating layer.

The insulating layer is to be a gate insulating layer later and can beformed by performing heat treatment (such as thermal oxidation treatmentor thermal nitridation treatment) on the surface of the semiconductorregion 104, for example. Instead of heat treatment, high-density plasmatreatment may be employed. The high-density plasma treatment can beperformed using, for example, a mixed gas of a rare gas such as He, Ar,Kr, or Xe and any of oxygen, nitrogen oxide, ammonia, nitrogen,hydrogen, and the like. Needless to say, the insulating layer may beformed by a CVD method, a sputtering method, or the like. The insulatinglayer preferably has a single-layer structure or a stacked structureincluding any of silicon oxide, silicon oxynitride, silicon nitride,hafnium oxide, aluminum oxide, tantalum oxide, yttrium oxide, hafniumsilicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)) to which nitrogen is added, hafnium aluminate (HfAl_(x)O_(y)(x>0, y>0)) to which nitrogen is added, and the like. The thickness ofthe insulating layer can be, for example, greater than or equal to 1 nmand less than or equal to 100 nm, preferably greater than or equal to 10nm and less than or equal to 50 nm.

The layer including a conductive material can be formed using a metalmaterial such as aluminum, copper, titanium, tantalum, or tungsten. Thelayer including a conductive material may be formed using asemiconductor material such as polycrystalline silicon. There is noparticular limitation on the formation method, and a variety of filmformation methods such as an evaporation method, a CVD method, asputtering method, and a spin coating method can be employed. Note thatan example in which the layer including a conductive material is formedusing a metal material is described in this embodiment.

After that, the insulating layer and the layer including a conductivematerial are selectively etched, so that the gate insulating layer 108and the gate electrode 110 are formed (see FIG. 16C).

Next, phosphorus (P), arsenic (As), or the like is added to thesemiconductor region 104, so that the channel formation region 116 andthe impurity regions 120 are formed (see FIG. 16D). Note that phosphorusor arsenic is added here in order to form an n-channel transistor; animpurity element such as boron (B) or aluminum (Al) may be added in thecase of forming a p-channel transistor. Here, the concentration of theimpurity added can be set as appropriate; the concentration ispreferably high when a semiconductor element is highly miniaturized.

Note that a sidewall insulating layer may be formed in the periphery ofthe gate electrode 110 so that an impurity region to which an impurityelement is added at a different concentration may be formed.

Then, a metal layer 122 is formed so as to cover the gate electrode 110,the impurity regions 120, and the like (see FIG. 17A). A variety of filmformation methods such as a vacuum evaporation method, a sputteringmethod, and a spin coating method can be employed for forming the metallayer 122. The metal layer 122 is preferably formed using a metalmaterial that reacts with a semiconductor material included in thesemiconductor region 104 to form a low-resistance metal compound.Examples of such a metal material include titanium, tantalum, tungsten,nickel, cobalt, and platinum.

Next, heat treatment is performed so that the metal layer 122 reactswith the semiconductor material. Thus, the metal compound regions 124that are in contact with the impurity regions 120 are formed (see FIG.17A). Note that in the case where the gate electrode 110 is formed usingpolycrystalline silicon or the like, a metal compound region is alsoformed in a portion of the gate electrode 110 in contact with the metallayer 122.

As the heat treatment, irradiation with a flash lamp can be employed,for example. Needless to say, another heat treatment method may be used;however, a method by which heat treatment for an extremely short timecan be achieved is preferably used in order to improve thecontrollability of chemical reaction in formation of the metal compound.Note that the metal compound regions are formed by reaction between themetal material and the semiconductor material and have sufficiently highconductivity. The formation of the metal compound regions can properlyreduce the electric resistance and improve element characteristics. Notethat the metal layer 122 is removed after the metal compound regions 124are formed.

Next, the electrode 126 is formed in a region in contact with part ofthe metal compound region 124 (see FIG. 17B). The electrode 126 isformed by, for example, forming a layer including a conductive materialand then selectively etching the layer. The layer including a conductivematerial can be formed using a metal material such as aluminum, copper,titanium, tantalum, or tungsten. The layer including a conductivematerial may be formed using a semiconductor material such aspolycrystalline silicon. There is no particular limitation on theformation method, and a variety of film formation methods such as anevaporation method, a CVD method, a sputtering method, and a spincoating method can be employed.

Then, the insulating layer 128 is formed so as to cover the componentsformed in the above steps (see FIG. 17C). The insulating layer 128 canbe formed using a material including an inorganic insulating materialsuch as silicon oxide, silicon oxynitride, silicon nitride, or aluminumoxide. In particular, a material with a low dielectric constant (a low-kmaterial) is preferably used for the insulating layer 128, becausecapacitance due to overlap of electrodes or wirings can be sufficientlyreduced. Note that the insulating layer 128 may be a porous insulatinglayer formed using any of those materials. A porous insulating layer hasa lower dielectric constant than a dense insulating layer, and thusallows a further reduction in capacitance due to electrodes or wirings.Further, the insulating layer 128 can be formed using an organicinsulating material such as polyimide or acrylic. Note that although theinsulating layer 128 has a single-layer structure here, one embodimentof the present invention is not limited to this. The insulating layer128 may have a stacked structure including two or more layers. In thecase of a three-layer structure, for example, a stacked structure of asilicon oxynitride layer, a silicon nitride oxide layer, and a siliconoxide layer can be employed.

Note that the electrode 126 can be formed so as to fill an opening whichis formed in the insulating layer 128 to reach the metal compound region124 after the formation of the insulating layer 128.

In that case, for example, it is possible to employ a method in which athin titanium film is formed in a region including the opening by a PVDmethod and a thin titanium nitride film is formed by a CVD method, andthen a tungsten film is formed so as to be embedded in the opening.Here, the titanium film formed by a PVD method has a function ofreducing an oxide film (such as a natural oxide film) formed on asurface over which the titanium film is formed, thereby lowering thecontact resistance with a lower electrode or the like (the metalcompound region 124, here). The titanium nitride film formed after theformation of the titanium film has a barrier function of preventingdiffusion of the conductive material. A copper film may be formed by aplating method after the formation of the barrier film of titanium,titanium nitride, or the like.

Through the above steps, the transistor 160 is formed with the use ofthe substrate 100 including a semiconductor material (see FIG. 17C).Thus, the stack 213 a can be formed. A feature of the transistor 160 isthat it can operate at high speed. Therefore, when the transistor isused as a reading transistor, data can be read at high speed.

After that, CMP treatment is performed on the insulating layer 128 aspretreatment for the formation of the transistor 162 and the capacitor164, so that upper surfaces of the gate electrode 110 and the electrode126 are exposed (see FIG. 17D). As treatment for exposing the uppersurfaces of the gate electrode 110 and the electrode 126, etchingtreatment or the like can be employed instead of CMP treatment; in orderto improve characteristics of the transistor 162, the surface of theinsulating layer 128 is preferably made as flat as possible.

Note that before or after the above steps, a step of forming anadditional electrode, wiring, semiconductor layer, insulating layer, orthe like may be performed. For example, a multilayer wiring structure inwhich an insulating layer and a conductive layer are stacked is employedas a wiring structure, whereby a highly integrated semiconductor devicecan be provided.

The above steps are described as a method for manufacturing thetransistor 160 as a transistor in the lower portion; at the time ofmanufacturing the transistor in the lower portion, the first drivercircuit 202 and the second driver circuit 203 illustrated in FIG. 3,FIG. 4, and the like can be manufactured.

<Method for Manufacturing Transistor in Upper Portion>

Next, a method for manufacturing the transistor 162 and the capacitor164 in the upper portion will be described with reference to FIGS. 18Ato 18D and FIGS. 19A and 19B.

First, an oxide semiconductor layer is formed over the gate electrode110, the electrode 126, the insulating layer 128, and the like and isprocessed, so that the oxide semiconductor layer 144 is formed (see FIG.18A). Note that an insulating layer functioning as a base may beprovided over the gate electrode 110, the electrode 126, and theinsulating layer 128 before the oxide semiconductor layer is formed. Theinsulating layer can be formed by a PVD method typified by a sputteringmethod, a CVD method such as a plasma CVD method, or the like.

As a material used for the oxide semiconductor layer, a four-componentmetal oxide such as an In—Sn—Ga—Zn—O-based material; a three-componentmetal oxide such as an In—Ga—Zn—O-based material, an In—Sn—Zn—O-basedmaterial, an In—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, or a Sn—Al—Zn—O-based material; atwo-component metal oxide such as an In—Zn—O-based material, aSn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-basedmaterial, a Sn—Mg—O-based material, an In—Mg—O-based material, or anIn—Ga—O-based material; an In—O-based material; a Sn—O-based material; aZn—O-based material; or the like can be used. In addition, the abovematerials may include SiO₂. Here, for example, an In—Ga—Zn—O-basedmaterial means an oxide semiconductor including indium (In), gallium(Ga), and zinc (Zn), and there is no particular limitation on thecomposition ratio thereof. Further, the In—Ga—Zn—O-based material mayinclude an element other than In, Ga, and Zn.

As the oxide semiconductor layer, a thin film including a materialexpressed as the chemical formula, InMO₃(ZnO)_(m) (m>0) can be used.Here, M represents one or more metal elements selected from Ga, Al, Mn,and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, orthe like.

The thickness of the oxide semiconductor layer is preferably greaterthan or equal to 3 nm and less than or equal to 30 nm. This is becausethe transistor might be normally on when the oxide semiconductor layeris too thick (e.g., the thickness is 50 nm or more).

The oxide semiconductor layer is preferably formed by a method withwhich impurities such as hydrogen, water, a hydroxyl group, and hydridedo not easily enter the oxide semiconductor layer. For example, asputtering method or the like can be used.

In this embodiment, the oxide semiconductor layer is formed by asputtering method using an In—Ga—Zn—O-based oxide target.

In the case where an In—Ga—Zn—O-based material is used as the oxidesemiconductor, for example, an oxide target having a composition ratioof In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can be used. Note that it is notnecessary to limit the material and composition ratio of the target tothe above. For example, an oxide target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] can be used.

In the case where an In—Zn—O-based material is used as the oxidesemiconductor, a target to be used has a composition ratio of In:Zn=50:1to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomicratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a targetused for formation of an In—Zn—O-based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, the relation Z>1.5X+Y is satisfied.

The filling rate of the target is higher than or equal to 90% and lowerthan or equal to 100%, preferably higher than or equal to 95% and lowerthan or equal to 99.9%. With the use of the target with a high fillingrate, the oxide semiconductor layer can be a dense film.

The deposition atmosphere may be a rare gas (typically argon)atmosphere, an oxygen atmosphere, a mixed atmosphere including a raregas and oxygen, or the like. Moreover, it is preferable to employ anatmosphere using a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, and hydride are sufficiently removedso that entry of hydrogen, water, a hydroxyl group, hydride, and thelike into the oxide semiconductor layer can be prevented.

For example, the oxide semiconductor layer can be formed as follows.

First, the substrate is held in a deposition chamber which is kept underreduced pressure, and then heating is performed so that the substratetemperature reaches a temperature higher than 200° C. and lower than orequal to 500° C., preferably higher than 300° C. and lower than or equalto 500° C., further preferably higher than or equal to 350° C. and lowerthan or equal to 450° C.

Then, a high-purity gas from which impurities such as hydrogen, water, ahydroxyl group, and hydride are sufficiently removed is introduced whilemoisture remaining in the deposition chamber is removed, and the oxidesemiconductor layer is formed over the substrate with the use of theabove target. In order to remove moisture remaining in the depositionchamber, an entrapment vacuum pump such as a cryopump, an ion pump, or atitanium sublimation pump is preferably used as an evacuation unit. Theevacuation unit may be a turbo pump provided with a cold trap. In thedeposition chamber which is evacuated with the cryopump, for example,impurities such as hydrogen, water, a hydroxyl group, and hydride(preferably, also a compound including a carbon atom) and the like areremoved, whereby the concentration of impurities such as hydrogen,water, a hydroxyl group, and hydride in the oxide semiconductor layerformed in the deposition chamber can be reduced.

In the case where the substrate temperature is low (e.g., 100° C. orlower) during deposition, a substance including a hydrogen atom mightenter the oxide semiconductor; thus, it is preferable that the substratebe heated at the above temperature. When the oxide semiconductor layeris formed with the substrate heated at the above temperature, thesubstrate temperature is high, so that hydrogen bonds are cut by heatand the substance including a hydrogen atom is less likely to be takeninto the oxide semiconductor layer. Therefore, the oxide semiconductorlayer is formed with the substrate heated at the above temperature,whereby the concentration of impurities such as hydrogen, water, ahydroxyl group, and hydride in the oxide semiconductor layer can besufficiently reduced. Moreover, damage due to sputtering can be reduced.

An example of the deposition conditions is as follows: the distancebetween the substrate and the target is 60 mm, the pressure is 0.4 Pa,the direct-current (DC) power is 0.5 kW, the substrate temperature is400° C., and the deposition atmosphere is an oxygen atmosphere (theproportion of oxygen flow is 100%). Note that a pulsed direct-currentpower source is preferable because powder substances (also referred toas particles or dust) generated in deposition can be reduced and thefilm thickness can be uniform.

Note that before the oxide semiconductor layer is formed by a sputteringmethod, powder substances (also referred to as particles or dust)attached to a surface over which the oxide semiconductor layer is formedare preferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which voltage is applied to a substrate to generate plasma inthe vicinity of the substrate so that a surface on the substrate side ismodified. Note that instead of argon, a gas such as nitrogen, helium, oroxygen may be used.

The oxide semiconductor layer can be processed by being etched after amask having a desired shape is formed over the oxide semiconductorlayer. The mask can be formed by a method such as photolithography.Alternatively, a method such as an inkjet method may be used to form themask. Note that the etching of the oxide semiconductor layer may be dryetching or wet etching. Needless to say, both of them may be employed incombination.

After that, heat treatment (first heat treatment) may be performed onthe oxide semiconductor layer 144. The heat treatment further removesthe substance including a hydrogen atom in the oxide semiconductor layer144; thus, a structure of the oxide semiconductor layer 144 can beimproved and defect levels in the energy gap can be reduced. The heattreatment is performed in an inert gas atmosphere at a temperaturehigher than or equal to 250° C. and lower than or equal to 700° C.,preferably higher than or equal to 450° C. and lower than or equal to600° C. or lower than a strain point of the substrate. The inert gasatmosphere is preferably an atmosphere which includes nitrogen or a raregas (such as helium, neon, or argon) as a main component and does notinclude water, hydrogen, and the like. For example, the purity ofnitrogen or a rare gas such as helium, neon, or argon introduced into aheat treatment apparatus is higher than or equal to 6N (99.9999%),preferably higher than or equal to 7N (99.99999%) (i.e., the impurityconcentration is lower than or equal to 1 ppm, preferably lower than orequal to 0.1 ppm).

The heat treatment can be performed in such a manner that, for example,an object to be heated is introduced into an electric furnace in which aresistance heating element or the like is used, and heated at 450° C.for one hour in a nitrogen atmosphere. The oxide semiconductor layer 144is not exposed to the air during the heat treatment so that entry ofwater and hydrogen can be prevented.

The above heat treatment can also be referred to as dehydrationtreatment, dehydrogenation treatment, or the like because of its effectof removing hydrogen, water, and the like. The heat treatment can beperformed at the timing, for example, before the oxide semiconductorlayer is processed into an island shape or after the gate insulatinglayer is formed. Such dehydration treatment or dehydrogenation treatmentmay be conducted once or plural times.

Next, a conductive layer for forming a source electrode and a drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the oxidesemiconductor layer 144 and the like and is processed, so that thesource or drain electrode 142 a and the source or drain electrode 142 bare formed (see FIG. 18B).

The conductive layer can be formed by a PVD method or a CVD method. As amaterial for the conductive layer, an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloyincluding any of these elements as a component; or the like can be used.A material including one of manganese, magnesium, zirconium, beryllium,neodymium, and scandium or a combination of any of these elements may beused.

The conductive layer may have a single-layer structure or a stackedstructure including two or more layers. For example, the conductivelayer can have a single-layer structure of a titanium film or a titaniumnitride film, a single-layer structure of an aluminum film includingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked, or thelike. Note that in the case where the conductive layer has asingle-layer structure of a titanium film or a titanium nitride film,there is an advantage in that the conductive layer is easily processedinto the source or drain electrode 142 a and the source or drainelectrode 142 b having tapered shapes.

Alternatively, the conductive layer may be formed using a conductivemetal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tinoxide (SnO₂), zinc oxide (ZnO), an indium oxide-tin oxide alloy(In₂O₃—SnO₂, which is abbreviated to ITO in some cases), an indiumoxide-zinc oxide alloy (In₂O₃—ZnO), or any of these metal oxidematerials including silicon or silicon oxide can be used.

The conductive layer is preferably etched so that end portions of thesource or drain electrode 142 a and the source or drain electrode 142 bare tapered. Here, a taper angle is, for example, preferably greaterthan or equal to 30° and less than or equal to 60°. The etching isperformed so that the end portions of the source or drain electrode 142a and the source or drain electrode 142 b are tapered, whereby coveragewith the gate insulating layer 146 formed later can be improved anddisconnection can be prevented.

The channel length (L) of the transistor in the upper portion isdetermined by the distance between a lower edge portion of the source ordrain electrode 142 a and a lower edge portion of the source or drainelectrode 142 b. Note that for light exposure for forming a mask used inthe case where a transistor with a channel length (L) of less than 25 nmis formed, it is preferable to use extreme ultraviolet light whosewavelength is as short as several nanometers to several tens ofnanometers. In the light exposure by extreme ultraviolet light, theresolution is high and the focus depth is large. Accordingly, thechannel length (L) of the transistor formed later can be greater than orequal to 10 nm and less than or equal to 1000 nm (1 μm), and theoperation speed of a circuit can be increased. Moreover, miniaturizationcan lead to lower power consumption of the semiconductor device.

Next, the gate insulating layer 146 is formed so as to cover the sourceor drain electrodes 142 a and 142 b and to be in contact with part ofthe oxide semiconductor layer 144 (see FIG. 18C).

The gate insulating layer 146 can be formed by a CVD method, asputtering method, or the like. The gate insulating layer 146 is formedusing a material such as silicon oxide, silicon nitride, or siliconoxynitride. Alternatively, the gate insulating layer 146 can be formedusing a material including a Group 13 element and oxygen. As thematerial including a Group 13 element and oxygen, for example, galliumoxide, aluminum oxide, aluminum gallium oxide, or the like can be used.Furthermore, the gate insulating layer 146 may be formed so as toinclude a high dielectric constant (high-k) material such as tantalumoxide, hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)), hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate to whichnitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)). The gateinsulating layer 146 may have a single-layer structure or a stackedstructure including a combination of any of the above materials. Thereis no particular limitation on the thickness; however, in the case wherethe semiconductor device is miniaturized, the thickness is preferablysmall for securing operation of the transistor. For example, in the casewhere silicon oxide is used, the thickness can be greater than or equalto 1 nm and less than or equal to 100 nm, preferably greater than orequal to 10 nm and less than or equal to 50 nm.

The gate insulating layer 146 is preferably formed by a method withwhich impurities such as hydrogen and water do not enter the gateinsulating layer 146. This is because, when impurities such as hydrogenand water are included in the gate insulating layer 146, the impuritiessuch as hydrogen and water enter the oxide semiconductor layer or oxygenin the oxide semiconductor layer is extracted by the impurities such ashydrogen and water, so that a back channel of the oxide semiconductorlayer might have lower resistance (have n-type conductivity) and aparasitic channel might be formed. Therefore, the gate insulating layer146 is preferably formed so as to include impurities such as hydrogenand water as few as possible. For example, the gate insulating layer 146is preferably formed by a sputtering method. A high-purity gas fromwhich impurities such as hydrogen and water are removed is preferablyused as a sputtering gas used in the deposition.

Many oxide semiconductor materials that can be used for the oxidesemiconductor layer 144 include a Group 13 element. Therefore, in thecase where the gate insulating layer 146 in contact with the oxidesemiconductor layer 144 is formed using a material including a Group 13element and oxygen, the state of the interface between the oxidesemiconductor layer 144 and the gate insulating layer 146 can be keptfavorable. This is because a material including a Group 13 element andoxygen is compatible with an oxide semiconductor material. For example,when the oxide semiconductor layer 144 and the gate insulating layer 146including gallium oxide are provided in contact with each other, pileupof hydrogen at the interface between the oxide semiconductor layer 144and the gate insulating layer 146 can be reduced. Aluminum oxide has aproperty of not easily transmitting water. Thus, it is preferable to usealuminum oxide for the gate insulating layer 146 in terms of preventingentry of water into the oxide semiconductor layer 144.

When the gate insulating layer is thin as described above, a problem ofgate leakage due to a tunneling effect or the like is caused. In orderto solve the problem of gate leakage, the above high-k material ispreferably used for the gate insulating layer 146. By using the high-kmaterial for the gate insulating layer 146, the thickness can beincreased to suppress gate leakage with electric characteristicsensured. Note that a stacked structure of a film including a high-kmaterial and a film including any of silicon oxide, silicon nitride,silicon oxynitride, silicon nitride oxide, aluminum oxide, and the likemay be employed.

In addition, the gate insulating layer 146 preferably includes oxygenmore than that in the stoichiometric composition. For example, whengallium oxide is used for the gate insulating layer 146, thestoichiometric composition can be expressed as Ga₂O_(3+α) (0<α<1). Whenaluminum oxide is used, the stoichiometric composition can be expressedas Al₂O_(3+α) (0<α<1). When gallium aluminum oxide is used, thestoichiometric composition can be expressed as Ga_(x)Al_(2−x)O_(3+α)(0<x<2, 0<α<1).

Note that oxygen doping treatment may be performed after deposition ofthe oxide semiconductor layer, after formation of the oxidesemiconductor layer 144, or after formation of the gate insulating layer146. The oxygen doping refers to addition of oxygen (which includes atleast one of an oxygen radical, an oxygen atom, and an oxygen ion) to abulk. Note that the term “bulk” is used in order to clarify that oxygenis added not only to a surface of a thin film but also to the inside ofthe thin film. In addition, “oxygen doping” includes “oxygen plasmadoping” in which oxygen that is made to be plasma is added to a bulk. Bythe oxygen doping treatment, oxygen can be included in the oxidesemiconductor layer or the gate insulating layer more than that in thestoichiometric composition.

The oxygen doping treatment is preferably performed by an inductivelycoupled plasma (ICP) method with the use of oxygen plasma which isexcited by a microwave (with a frequency of 2.45 GHz, for example).

After the gate insulating layer 146 is formed, second heat treatment ispreferably performed in an inert gas atmosphere or an oxygen atmosphere.The temperature of the heat treatment is higher than or equal to 200° C.and lower than or equal to 450° C., preferably higher than or equal to250° C. and lower than or equal to 350° C. For example, the heattreatment may be performed at 250° C. for one hour in a nitrogenatmosphere. The second heat treatment can reduce variation in electriccharacteristics of the transistor. Further, in the case where the gateinsulating layer 146 includes oxygen, oxygen can be supplied to theoxide semiconductor layer 144 to cover oxygen deficiency in the oxidesemiconductor layer 144.

Note that in this embodiment, the second heat treatment is performedafter the gate insulating layer 146 is formed; however, the timing ofthe second heat treatment is not limited to this. For example, thesecond heat treatment may be performed after a gate electrode is formed.Alternatively, the first heat treatment and the second heat treatmentmay be successively performed, the first heat treatment may also serveas the second heat treatment, or the second heat treatment may alsoserve as the first heat treatment.

By performing at least one of the first heat treatment and the secondheat treatment as described above, the oxide semiconductor layer 144 canbe highly purified so as to include the substance including a hydrogenatom as few as possible.

Next, a conductive layer for forming a gate electrode (including awiring formed in the same layer as the gate electrode) is formed andprocessed, so that the gate electrode 148 a and the conductive layer 148b are formed (see FIG. 18D).

The gate electrode 148 a and the conductive layer 148 b can be formedusing a metal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy material includingany of these materials as a main component. The gate electrode 148 a andthe conductive layer 148 b may each have a single-layer structure or astacked structure.

Next, the insulating layer 150 and the insulating layer 152 are formedover the gate insulating layer 146, the gate electrode 148 a, and theconductive layer 148 b (see FIG. 19A). The insulating layer 150 and theinsulating layer 152 can be formed by a PVD method, a CVD method, or thelike. The insulating layer 150 and the insulating layer 152 can beformed using a material including an inorganic insulating material suchas silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide,gallium oxide, aluminum oxide, or gallium aluminum oxide. Note that theinsulating layer 150 and the insulating layer 152 are preferably formedusing a low dielectric constant material or to have a structure with alow dielectric constant (such as a porous structure). This is because byreducing the dielectric constant of the insulating layer 150 and theinsulating layer 152, capacitance between wirings, electrodes, or thelike can be reduced; thus, operation at higher speed can be achieved.Note that although the insulating layer 150 and the insulating layer 152each have a single-layer structure in this embodiment, one embodiment ofthe present invention is not limited to this. The insulating layer 150and the insulating layer 152 may each have a stacked structure includingtwo or more layers.

Then, an opening 153 reaching the source or drain electrode 142 b isformed in the gate insulating layer 146, the insulating layer 150, andthe insulating layer 152. After that, the electrode 154 which is incontact with the source or drain electrode 142 b is formed in theopening 153, and the wiring 156 which is in contact with the electrode154 is formed over the insulating layer 152 (see FIG. 19B). Thus, thestack 213 b can be formed. Note that the opening is formed by selectiveetching using a mask or the like.

The electrode 154 can be formed in such a manner that, for example, aconductive layer is formed by a PVD method, a CVD method, or the like ina region including the opening 153 and then part of the conductive layeris removed by etching treatment, CMP treatment, or the like.

Specifically, for example, it is possible to employ a method in which athin titanium film is formed by a PVD method in a region including theopening 153 and a thin titanium nitride film is formed by a CVD method,and then a tungsten film is formed so as to be embedded in the opening153. Here, the titanium film formed by a PVD method has a function ofreducing an oxide film (such as a natural oxide film) formed on asurface over which the titanium film is formed, thereby lowering thecontact resistance with a lower electrode or the like (the source ordrain electrode 142 b, here). The titanium nitride film formed after theformation of the titanium film has a barrier function of preventingdiffusion of the conductive material. A copper film may be formed by aplating method after the formation of the barrier film of titanium,titanium nitride, or the like.

Note that in the case where the electrode 154 is formed by removing partof the conductive layer, processing is preferably performed so that thesurface is planarized. For example, in the case where a thin titaniumfilm or a thin titanium nitride film is formed in a region including theopening 153 and then a tungsten film is formed so as to be embedded inthe opening 153, excessive tungsten, titanium, titanium nitride, or thelike can be removed and the planarity of the surface can be improved bysubsequent CMP treatment. The surface including the electrode 154 isplanarized in this manner, whereby an electrode, a wiring, an insulatinglayer, a semiconductor layer, or the like can be favorably formed in alater step.

The wiring 156 is formed in such a manner that a conductive layer isformed by a PVD method typified by a sputtering method or a CVD methodsuch as a plasma CVD method and then the conductive layer is patterned.As a material for the conductive layer, an element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy including any of these elements as a component; orthe like can be used. A material including one of manganese, magnesium,zirconium, beryllium, neodymium, and scandium or a combination of any ofthese elements may be used. The details are similar to those of thesource or drain electrodes 142 a and 142 b, and the like.

Through the above steps, the first stack 210 a including the transistor160, the transistor 162, and the capacitor 164 is completed (see FIG.19B).

Next, the insulating layer 158 is formed over the first stack 210 a, andthe second stack 210 b is formed over the insulating layer 158 (see FIG.1). The second stack 210 b includes the transistor 170, the transistor172, and the capacitor 174. Here, a method for manufacturing thetransistor 170, the transistor 172, and the capacitor 174 is similar tothe method for manufacturing the transistor 162 and the capacitor 164,and thus is not described in detail. In the case where a third stack ora fourth stack is formed over the second stack 210 b, a transistor and acapacitor which are similar to the transistor 170, the transistor 172,and the capacitor 174 may be formed with an insulating layer positionedbetween the second stack 210 b and the transistor and the capacitor.

In a manufacturing process of a transistor including an oxidesemiconductor layer, high-temperature treatment is not needed and thusthe transistor can be manufactured without affecting a device or awiring in a first stack. Further, the manufacturing process of thetransistor including an oxide semiconductor layer has a smaller numberof steps than a manufacturing process of a transistor including asemiconductor material (e.g., silicon) other than an oxidesemiconductor. Accordingly, by using a stack formed using the transistorincluding an oxide semiconductor layer as a second stack or a thirdstack, the manufacturing cost can be reduced.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiment.

Embodiment 2

In this embodiment, application of the semiconductor device described inthe above embodiment to an electronic device will be described withreference to FIGS. 20A to 20F. In this embodiment, examples of theelectronic device to which the above semiconductor device is appliedinclude a computer, a mobile phone (also referred to as a cellular phoneor a mobile phone device), a portable information terminal (including aportable game machine, an audio reproducing device, and the like), acamera such as a digital camera or a digital video camera, electronicpaper, and a television device (also referred to as a television or atelevision receiver).

FIG. 20A illustrates a laptop computer including a housing 701, ahousing 702, a display portion 703, a keyboard 704, and the like. Amemory circuit is provided inside at least one of the housings 701 and702, and the memory circuit includes the semiconductor device describedin the above embodiment. Therefore, a laptop computer in which writingand reading of data are performed at high speed, data is held for a longtime, and power consumption is sufficiently reduced can be realized.

FIG. 20B illustrates a portable information terminal (PDA). A main body711 is provided with a display portion 713, an external interface 715,an operation button 714, and the like. Further, a stylus 712 foroperation of the portable information terminal, or the like is provided.A memory circuit is provided inside the main body 711, and the memorycircuit includes the semiconductor device described in the aboveembodiment. Therefore, a portable information terminal in which writingand reading of data are performed at high speed, data is held for a longtime, and power consumption is sufficiently reduced can be realized.

FIG. 20C illustrates an electronic book reader 720 incorporatingelectronic paper, which includes two housings, a housing 721 and ahousing 723. The housing 721 and the housing 723 are provided with adisplay portion 725 and a display portion 727, respectively. Thehousings 721 and 723 are connected by a hinge 737 and can be opened andclosed using the hinge 737 as an axis. The housing 721 is provided witha power switch 731, an operation key 733, a speaker 735, and the like. Amemory circuit is provided inside at least one of the housings 721 and723, and the memory circuit includes the semiconductor device describedin the above embodiment. Therefore, an electronic book reader in whichwriting and reading of data are performed at high speed, data is heldfor a long time, and power consumption is sufficiently reduced can berealized.

FIG. 20D illustrates a mobile phone including two housings, a housing740 and a housing 741. Further, the housing 740 and the housing 741which are in a state where they are developed as illustrated in FIG. 20Dcan slide so that one is lapped over the other. Thus, the mobile phonecan be made small and suitable for being carried. The housing 741includes a display panel 742, a speaker 743, a microphone 744, anoperation key 745, a pointing device 746, a camera lens 747, an externalconnection terminal 748, and the like. The housing 740 includes a solarcell 749 for charging the mobile phone, an external memory slot 750, andthe like. In addition, an antenna is incorporated in the housing 741. Amemory circuit is provided inside at least one of the housings 740 and741, and the memory circuit includes the semiconductor device describedin the above embodiment. Therefore, a mobile phone in which writing andreading of data are performed at high speed, data is held for a longtime, and power consumption is sufficiently reduced can be realized.

FIG. 20E illustrates a digital camera including a main body 761, adisplay portion 767, an eyepiece 763, an operation switch 764, a displayportion 765, a battery 766, and the like. In the main body 761, thesemiconductor device described in the above embodiment is provided.Therefore, a digital camera in which writing and reading of data areperformed at high speed, data is held for a long time, and powerconsumption is sufficiently reduced can be realized.

FIG. 20F illustrates a television set 770 including a housing 771, adisplay portion 773, a stand 775, and the like. The television set 770can be operated with a switch of the housing 771 or a remote controller780. A memory circuit is provided inside the housing 771 and the remotecontroller 780, and the memory circuit includes the semiconductor devicedescribed in the above embodiment. Therefore, a television set in whichwriting and reading of data are performed at high speed, data is heldfor a long time, and power consumption is sufficiently reduced can berealized.

As described above, the electronic devices described in this embodimenteach include the semiconductor device according to the above embodiment.Therefore, electronic devices with low power consumption can berealized.

EXPLANATION OF REFERENCE

-   -   100: substrate, 102: protective layer, 104: semiconductor        region, 106: element isolation insulating layer, 108: gate        insulating layer, 110: gate electrode, 116: channel formation        region, 120: impurity region, 122: metal layer, 124: metal        compound region, 126: electrode, 128: insulating layer, 142 a:        source or drain electrode, 142 b: source or drain electrode,        144: oxide semiconductor layer, 146: gate insulating layer, 148        a: gate electrode, 148 b: conductive layer, 150: insulating        layer, 152: insulating layer, 153: opening, 154: electrode, 156:        wiring, 158: insulating layer, 160: transistor, 162: transistor,        164: capacitor, 170: transistor, 172: transistor, 174:        capacitor, 201: memory cell array, 202: first driver circuit,        203: second driver circuit, 210 a: first stack, 210 b: second        stack, 210 c: third stack, 211 a: memory cell array, 211 b:        memory cell array, 211 c: memory cell array, 212 a: memory cell,        212 b: memory cell, 212 c: memory cell, 213 a: stack, 213 b:        stack, 213 c: stack, 213 d: stack, 221: selector, 221 a:        selector, 221 b: selector, 221 c: selector, 222: circuit, 223:        row decoder, 231: selector, 231 a: selector, 231 b: selector,        231 c: selector, 232: circuit group, 232 a: circuit group, 232        b: circuit group, 233: column decoder, 234: circuit group, 235:        circuit group, 236: register group, 237: circuit, 238: circuit,        239: register, 701: housing, 702: housing, 703: display portion,        704: keyboard, 711: main body, 712: stylus, 713: display        portion, 714: operation button, 715: external interface, 720:        electronic book reader, 721: housing, 723: housing, 725: display        portion, 727: display portion, 731: power switch, 733: operation        key, 735: speaker, 737: hinge, 740: housing, 741: housing, 742:        display panel, 743: speaker, 744: microphone, 745: operation        key, 746: pointing device, 747: camera lens, 748: external        connection terminal, 749: solar cell, 750: external memory slot,        761: main body, 763: eyepiece, 764: operation switch, 765:        display portion, 766: battery, 767: display portion, 770:        television set, 771: housing, 773: display portion, 775: stand,        and 780: remote controller.

This application is based on Japanese Patent Application serial no.2010-152021 filed with the Japan Patent Office on Jul. 2, 2010, theentire contents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: asubstrate; a first transistor over the substrate; a second transistorover the substrate, and a capacitor electrically connected to the firsttransistor and the second transistor, wherein the first transistor andthe second transistor are at least partially overlapped with each otherwith an insulating layer interposed there between, wherein each of thefirst transistor and the second transistor comprises a channel formationregion comprising an oxide semiconductor, wherein a gate of the firsttransistor is directly connected to one of a source or a drain of thesecond transistor, wherein the capacitor comprises a first electrode, asecond electrode, and a first insulating layer provided between thefirst electrode and the second electrode, and wherein the one of thesource or the drain of the second transistor is the first electrode ofthe capacitor.
 2. The semiconductor device according to claim 1, whereinthe oxide semiconductor comprises indium.
 3. The semiconductor deviceaccording to claim 1, wherein one of a source or a drain of the firsttransistor is connected to the other one of the source or the drain ofthe second transistor.
 4. The semiconductor device according to claim 1,wherein the oxide semiconductor contains hydrogen at a concentrationlower than or equal to 5×10¹⁹ atoms/cm³.
 5. The semiconductor deviceaccording to claim 1, wherein the oxide semiconductor contains carriersat a concentration lower than or equal to 1×10¹²/cm³.
 6. Thesemiconductor device according to claim 1, wherein off-state current permicrometer of a channel width of the second transistor is lower than orequal to 100 zA at room temperature.
 7. The semiconductor deviceaccording to claim 1, wherein a top surface of the gate of the firsttransistor is directly connected to a bottom surface of the one of thesource or the drain of the second transistor.
 8. A semiconductor devicecomprising: a driver circuit comprising a first transistor, the firsttransistor comprising a channel formation region which comprises singlecrystal silicon; a first insulating layer over the first transistor; asecond transistor over the first insulating layer; a second insulatinglayer over the second transistor; a third transistor over the secondinsulating layer, and a capacitor electrically connected to the secondtransistor and the third transistor, wherein each of the secondtransistor and the third transistor comprises a channel formation regioncomprising an oxide semiconductor, wherein a gate of the secondtransistor is directly connected to one of a source or a drain of thethird transistor, wherein the second transistor and the third transistorare at least partially overlapped with each other, wherein the capacitorcomprises a first electrode, a second electrode, and a first insulatinglayer provided between the first electrode and the second electrode, andwherein the one of the source or the drain of the third transistor isthe first electrode of the capacitor.
 9. The semiconductor deviceaccording to claim 8, wherein the oxide semiconductor comprises indium.10. The semiconductor device according to claim 8, wherein one of asource or a drain of the second transistor is connected to the other oneof the source or the drain of the third transistor.
 11. Thesemiconductor device according to claim 8, wherein the driver circuitcomprises a selector circuit.
 12. The semiconductor device according toclaim 8, wherein the channel formation region of the first transistor isformed in a single crystal silicon substrate.
 13. The semiconductordevice according to claim 8, wherein the oxide semiconductor containshydrogen at a concentration lower than or equal to 5×10¹⁹ atoms/cm³. 14.The semiconductor device according to claim 8, wherein the oxidesemiconductor contains carriers at a concentration lower than or equalto 1×10¹²/cm³.
 15. The semiconductor device according to claim 8,wherein off-state current per micrometer of a channel width of the thirdtransistor is lower than or equal to 100 zA at room temperature.
 16. Thesemiconductor device according to claim 8, wherein a top surface of thegate of the second transistor is directly connected to a bottom surfaceof the one of the source or the drain of the third transistor.
 17. Asemiconductor device comprising: a substrate provided with a drivercircuit, the substrate comprising single crystal silicon; a firstinsulating layer over the substrate; a first transistor over the firstinsulating layer; a second insulating layer over the first transistor; asecond transistor over the second insulating layer; and a capacitorelectrically connected to the first transistor and the secondtransistor, wherein each of the first transistor and the secondtransistor comprises a channel formation region comprising an oxidesemiconductor, wherein a gate of the first transistor is directlyconnected to one of a source or a drain of the second transistor, andwherein the first transistor and the second transistor are at leastpartially overlapped with each other, wherein the capacitor comprises afirst electrode, a second electrode, and a first insulating layerprovided between the first electrode and the second electrode, andwherein the one of the source or the drain of the second transistor isthe first electrode of the capacitor.
 18. The semiconductor deviceaccording to claim 17, wherein the oxide semiconductor comprises indium.19. The semiconductor device according to claim 17, wherein one of asource or a drain of the first transistor is connected to the other oneof the source or the drain of the second transistor.
 20. Thesemiconductor device according to claim 17, wherein the driver circuitcomprises a selector circuit.
 21. The semiconductor device according toclaim 17, wherein the oxide semiconductor contains hydrogen at aconcentration lower than or equal to 5×10¹⁹ atoms/cm³.
 22. Thesemiconductor device according to claim 17, wherein the oxidesemiconductor contains carriers at a concentration lower than or equalto 1×10¹²/cm³.
 23. The semiconductor device according to claim 17,wherein off-state current per micrometer of a channel width of thesecond transistor is lower than or equal to 100 zA at room temperature.24. The semiconductor device according to claim 17, wherein a topsurface of the gate of the first transistor is directly connected to abottom surface of the one of the source or the drain of the secondtransistor.
 25. A semiconductor device comprising: a substratecomprising single crystal silicon; a first memory cell over thesubstrate, the first memory cell comprising a first transistor; aninsulating layer over the substrate; and a second memory cell over theinsulating layer, the second memory cell comprising a second transistorand a capacitor, wherein each of the first transistor and the secondtransistor comprises a channel formation region comprising an oxidesemiconductor, wherein the first transistor and the second transistorare at least partially overlapped with each other, wherein the capacitorcomprises a first electrode, a second electrode, and a first insulatinglayer provided between the first electrode and the second electrode, andwherein one of a source or a drain of the second transistor is the firstelectrode of the capacitor.
 26. The semiconductor device according toclaim 25, wherein a driver circuit comprising a transistor is providedbetween the substrate and the first memory cell, and wherein a channelformation region of the transistor is formed in the substrate.
 27. Thesemiconductor device according to claim 25, wherein the oxidesemiconductor comprises indium.
 28. The semiconductor device accordingto claim 25, wherein the oxide semiconductor contains hydrogen at aconcentration lower than or equal to 5×10¹⁹ atoms/cm³.
 29. Thesemiconductor device according to claim 25, wherein the oxidesemiconductor contains carriers at a concentration lower than or equalto 1×10¹²/cm³.
 30. The semiconductor device according to claim 25,wherein off-state current per micrometer of a channel width of thesecond transistor is lower than or equal to 100 zA at room temperature.31. A semiconductor device comprising: a first transistor comprising afirst oxide semiconductor layer; a second transistor comprising a secondoxide semiconductor layer; and a capacitor electrically connected to thefirst transistor and the second transistor, wherein an insulating layeris located between the first oxide semiconductor layer and the secondoxide semiconductor layer, wherein a gate of the first transistor isdirectly connected to a source or a drain of the second transistor,wherein the capacitor comprises a first electrode, a second electrode,and a first insulating layer provided between the first electrode andthe second electrode, and wherein the source or the drain of the secondtransistor is the first electrode of the capacitor.
 32. Thesemiconductor device according to claim 31, wherein the second oxidesemiconductor layer is located above the insulating layer.
 33. Thesemiconductor device according to claim 31, wherein the insulating layeris a gate insulating layer of the first transistor.
 34. Thesemiconductor device according to claim 31, wherein one of a source or adrain of the first transistor is connected to the other one of thesource or the drain of the second transistor.
 35. The semiconductordevice according to claim 31, wherein the first oxide semiconductorlayer comprises indium.
 36. The semiconductor device according to claim31, wherein the first oxide semiconductor layer contains hydrogen at aconcentration lower than or equal to 5×10¹⁹ atoms/cm³.
 37. Thesemiconductor device according to claim 31, wherein the first oxidesemiconductor layer contains carriers at a concentration lower than orequal to 1×10¹²/cm³.
 38. The semiconductor device according to claim 31,wherein off-state current per micrometer of a channel width of thesecond transistor is lower than or equal to 100 zA at room temperature.39. The semiconductor device according to claim 31, wherein a topsurface of the gate of the first transistor is directly connected to abottom surface of the source or the drain of the second transistor.